Stream Dynamics Fact Sheet - May 2003

Stream Dynamics Fact Sheet

Streambank erosion is a common resource concern seen
in waterways throughout the state. The results--degraded water quality, unstable
structures, and increased flood risks--impact communities environmentally and
economically. Understanding stream dynamics is crucial in order to stabilize and
maintain Illinois’ streambanks.

Understanding Streambank Erosion

Natural streams change and adjust their shape and pattern in response to the
speed, volume and duration of flow over long time periods. The way that natural
forces interact to shift and alter stream patterns and characteristics is
described as stream dynamics. A good understanding of stream dynamics in
Illinois first requires recognition of how these natural forces (e.g. stream
flow, sediment load) have been altered.

Over many years, streams develop their patterns and characteristics of
transporting water and sediment from upland areas, through floodplains, and on
to larger streams and rivers, and eventually, oceans. For hundreds of years,
only minor changes were made to the landscape, and stream systems developed a
balance of size and shape capable of carrying the water and sediment generated
within each watershed. This balance is known as a state of equilibrium in which
stream channels continue to shift and change slowly while maintaining their
overall shape and size.

The Effects of Human Activity on Stream Dynamics

The Illinois landscape has been altered dramatically by human activity since
the beginning of European settlement over 200 years ago. We have cleared the
timber and plowed the prairie, drained the wetlands and straightened the creeks,
to develop some of the most productive farmland in the world. In the process, we
also have covered large areas of our cities with concrete, asphalt and
rooftops—impermeable surfaces that cannot absorb rainfall as the forests and
prairies once did. The development of both farmland and urban land has resulted
in a landscape that produces more runoff that moves at a faster rate.
“Straightened” creeks then quickly transport the runoff at increased velocities.
Finally, we have built levees along the streambanks to confine all the water in
the smallest area possible. It is easy to recognize that the natural forces
which cause streams to adjust and change their shapes and patterns have
themselves been altered. These alterations are causing our stream systems to
change dramatically in an attempt to restore equilibrium.

The response of a stream to watershed changes has been expressed by hydraulic
engineer E.W. Lane as a stream balance equation (Lane, 1955). Lane concluded
that a stream’s energy, a function of speed and volume of water, must be in
balance with the size and volume of sediment carried by the stream. In practical
terms, this means that if either the volume of water (increased runoff) or
velocity of water (steeper slope usually caused by channelization) increases,
then the stream will need to carry more sediment to balance the increased
energy. The usual source for the additional sediment is either from the stream
bottom or the stream banks—resulting in severe erosion. Conversely, if sediment
load exceeds the available energy to transport it, then the stream aggrades, or
fills in, causing loss of capacity and increased flooding. Both conditions are
constantly observed in Illinois streams.

From Disruption to Equilibrium

Once the stream equilibrium has been disrupted, the stream bottom typically
erodes, deepening the channel. This process is referred to as downcutting. This
in turn sets off a series of events that is described by a Channel Evolution
Model (CEM) (Simon, 1989). Simply put, the CEM describes that as a channel cuts
deeper, more water will remain inside the deeper channel before the stream
floods, further increasing velocity and setting off a series of events that will
result in failing banks, widening of the top stream width and development of a
floodplain within the new channel. After this series of events, the channel will
have again established equilibrium, but at a lower elevation and with a newly
formed floodplain. It is important to understand that once a channel downcuts at
one location, it will downcut upstream and in all tributaries throughout the
watershed unless a grade control is encountered (i.e. bedrock or an artificial
structure, such as a culvert). Therefore, if a lower reach undergoes downcutting
as the result of a channelization project, the effects will be felt throughout
the watershed, although it may take many years or decades for the effects of the
downcutting to be seen throughout the entire watershed.

Stabilizing streams is a matter of balancing the force of the flowing water
against the resistance of the channel sides and bottom. This task may sound
simple but can be very complex and costly. Left alone, nature will find this
balance; however, it may take decades, and may cause tremendous damage to
cropland, roads, homes and other structures built near the stream. Also, large
deposits of sediment may result in streams, lakes, and oceans that may degrade
aquatic habitat and require expensive maintenance.

Maintaining Equilibrium

Is there a solution? Yes! Newer, less costly, and more environmentally
friendly methods of stream stabilization known as “Soil Bioengineering,” a blend
of natural processes and traditional engineering solutions, have been developed
and tested. The key to successful stabilization is an understanding of the
natural processes that are causing destabilization at each location. Once the
cause of the problem is determined, then a low-cost, innovative solution may be
found. Individuals interested in receiving technical assistance in the
assessment and treatment of streambank erosion problems should contact their
local NRCS or SWCD office.

Certain types of stream work require permits. For more information, contact your
U.S. Army Corps of Engineers, District Regulatory Office; and the Illinois
Department of Natural Resources, Office of Water Resources.